The present invention relates to an implant having a shaft which is adapted in use to be embedded in bone tissue and which has an outer surface provided with a circumferentially-oriented roughness. This will hereinafter be referred to as an xe2x80x9cimplant of the type definedxe2x80x9d.
Implants of the type defined are known for use as the anchoring members of dental and orthopaedic prostheses. To this end, the implant is inserted into a bore-hole drilled into the bone tissue of a bone tissue structure at a site where a prosthesis is required, ordinarily by screwing of the implant into the bore-hole. The convention in the art is for the circumferentially-oriented roughness to take the form of a screw thread and in this case the bore-hole will ordinarily be (i) provided with internal threads in advance, or (ii) left untapped with the implant being provided with a self-tapping capacity, e.g. by the provision of one or more axially-extending cutting recesses or notches in the screw thread.
A superstructure having the prosthetic part of the prosthesis is then secured to the implant. In the case of a dental prosthesis, the superstructure will typically consist of a spacer or transmucosal component which engages to the implant to bridge the gingiva overlying the maxilla or mandible at the implant site and the prosthetic part, e.g. a crown, bridge or denture, is then secured to the spacer. There are various other forms that the superstructure can take as is known in the art. For instance, the prosthetic part may be secured directly to the implant.
The long-term integrity of the prosthesis is highly dependent on the successful osseointegration of the implant with the bone tissue structure, that is to say, the remodelling of the bone tissue in the bone tissue structure into direct apposition with the implant. A study on the factors which affect the osseointegration of implants was undertaken by Professor Per-Ingvar Br{dot over (a)}nemark and co-workers and the results were published in a book entitled xe2x80x9cOsseointegrated Implants in the Treatment of the Edentulous law: Experience from a 10-Year Periodxe2x80x9d, Almqvist and Wiskell International. Stockholm, Sweden, 1977. It was found by Br{dot over (a)}nemark et al that successful osseointegration depends upon inter alia the use of biocompatible materials for the implant, for example titanium and alloys thereof, and the surgical procedure adopted, for example leaving the implant unloaded for several months before adding the superstructure. Implants of the type defined are not necessarily always used as part of a prosthesis, in some instances they can be a xe2x80x9cstand alonexe2x80x9d structure. As an example, implants of the type defined are known for use as bone fixation screws. The success of these xe2x80x9cstand alonexe2x80x9d implants is also highly dependent on their successful osseointegration.
Implants of the type defined have some notable advantages in promoting successful osseointegration with the adjacent bone tissue, a major one being as a result of the fact that the main loads on the implant in the clinical situation are axial loads. These implants are very well suited to support axial loads and this may be particularly important in the initial stages of the osseointegration process in which it is important that the implant is fully stable and as immovable as possible in the bore-hole (primary fixation). One can consider this to be due to the bone tissue growing into the troughs between adjacent peaks of the circumferentially-oriented roughness on the implant.
The Applicant has also identified that it is advantageous for an implant of the type defined to transmit the axial loading thereon evenly to the adjacent bone tissue to prevent high stress concentrations occurring in the adjacent bone tissue and concomitantly marginal bone tissue resorption. If marginal bone tissue resorption occurs this will reduce the anchorage of the implant and may undermine the long term stability of the implant resulting in due course in failure of the prosthesis. In the particular case of dental prostheses, the aesthetic appeal is also undermined by marginal bone tissue resorption, an important drawback since dental prosthetics forms part of the field of cosmetic surgery.
The present invention proposes to provide an implant of the type defined having features which promote its maintenance in a bone tissue structure whilst at the same time facilitating its insertion into the bone tissue structure in the first place.
According to the present invention there is provided an implant of the type defined in which the circumferentially-oriented roughness has first and second axial sections each comprising a series of circumferentially-oriented peaks which have a crest and which are axially spaced apart by troughs, the axial spacing between the crests of adjacent peaks in the first axial section is less than the axial spacing between the crests of adjacent peaks in the second axial section and the first and second axial sections of circumferentially-oriented roughness are adapted in use to provide the same or substantially the same pitch.
The larger inter-peak spacing in the second axial section of circumferentially-oriented roughness acts to promote primary fixation of the implant in the bone tissue during the early phases of osseointegration since each trough between adjacent peaks can capture a relatively large volume of bone tissue to interlock the implant with the bone tissue. The smaller inter-peak spacing in the first axial section, on the other hand, enables the stiffness of the implant to be increased thereby improving the ability of the implant to transmit loads more evenly to the bone tissue to inhibit marginal bone resorption. Adapting the first and second axial sections to have the same or substantially the same pitch means that both axial sections produce the same or substantially the same axial displacement into the bone tissue on one rotation thereof thus ensuring that the provision of the two different axial sections of circumferentially-oriented roughness does not complicate insertion of the implant into the bone tissue. If the first and second axial sections of circumferentially-oriented roughness did not have the same or substantially the same pitch then a greater force would need to be applied to insert the implant resulting in fractures being formed in the bone tissue.
In an embodiment of the invention such as the one hereinafter to be described the pitch is a predetermined distance, the ratio of the predetermined distance to the axial spacing between the crests of adjacent peaks in the first axial section is a first multiple integer and the ratio of the predetermined distance to the axial spacing between the crests of adjacent peaks in the second axial section is a second multiple integer which is less than the first multiple integer. The first multiple integer may be a multiple integer of the second multiple integer.
In an embodiment of the invention such as the one hereinafter to be described the peaks in the first and second axial sections are circumferentially-oriented at a common inclined angle to the main axis of the implant.
In an embodiment of the invention such as the one hereinafter to be described the shaft has a coronal end and an apical end and the first axial section is located coronally of the second axial section.
In an embodiment of the invention such as the one hereinafter to be described the first and second axial sections are contiguous.
In an embodiment of the invention such as the one hereinafter to be described the first axial section extends from the coronal end of the shaft to a position coronally of the apical end and the second axial section extends from the first axial section towards the apical end of the shaft. The implant may have a coronal end which is spaced coronally from the coronal end of the shaft by a smooth coronal portion of the implant, as in the embodiment of the invention hereinafter to be described, in which case the smooth coronal portion is preferably no more than 4% of the total length of the implant, more preferably in the range 1.5-3.7% of said total length.
In an embodiment of the invention such as the one hereinafter to be described the axial extent of the first axial section is greater than the axial extent of the second axial section. Alternatively, the axial extent of the first axial section may be less than the axial extent of the second axial section or the axial extents of the first and second axial sections may be the same or substantially the same.
In an embodiment of the invention in which the first axial section is disposed coronally of the second axial section, such as the one hereinafter to be described, a blind bore extends apically into the shaft from the coronal end thereof to an end surface in-between the apical and coronal ends of the shaft for a superstructure to be secured to the implant, the blind bore comprising an internally-threaded section having a coronal edge and an apical edge for screw connection of the superstructure to the implant with the apical edge terminating at a position which is disposed apically of the first axial section. Alternately, the apical edge of the internally-threaded section of the blind bore may terminate at a position which is disposed coronally of the second axial section. The internally-threaded section may be an apical section of the blind bore, as in the embodiment of the invention hereinafter to be described.
In an embodiment of the invention such as the one hereinafter to be described all or substantially all of the crests of the peaks in the first and second axial sections lie on an axial plane parallel to the main axis of the shaft. Expressed another way, the major transverse dimension of the implant at the first and second axial sections is uniform.
In an embodiment of the invention such as the one hereinafter to be described the height of the peaks, as measured from the troughs to the crests, in the first axial section differs from that in the second axial section. To advantage, the height of the peaks in the first axial section is less than that in the second axial section. This feature further enables the stiffness of the implant to be increased.
In an alternative embodiment of the invention the height of the peaks, as measured from the troughs to the crests, in the first axial section is the same or substantially the same as in the second axial section.
In an embodiment of the invention such as the one hereinafter to be described the ratio of the height of the peaks, as measured from the troughs to the crests, to the axial spacing between the crests of adjacent peaks in the first axial section is the same or substantially the same as in the second axial section.
In an embodiment of the invention such as the one hereinafter to be described the height of the peaks, as measured from the troughs to the crests, in the first axial section is no greater than 0.2 mm, for example in the range 0.02-0.20 mm, and the height of the peaks, as measured from the troughs to the crests, in the second axial section is greater than that in the first axial section, for instance in the range 0.15 mm to 1 mm. Such heights complement the primary fixation and stiffness characteristics of the implant provided by the different inter-peak spacings of the first and second axial sections.
In an embodiment of the invention such as the one hereinafter to be described the peaks in the first and second axial sections are bounded by flank surfaces and the angle between the opposed flanks of adjacent peaks in the first and second axial sections is the same.
In an embodiment of the invention such as the one hereinafter to be described the troughs in at least one of the first and second axial sections are a continuous curved surface.
In an embodiment of the invention such as the one hereinafter to be described the circumferentially-oriented roughness in the first and/or second axial section is presented by a screw thread profile with the circumferentially-oriented peaks being defined by thread elements of the screw thread profile.
Typically, the screw thread profile of the first and/or second axial section will be formed by a screw thread structure. In such case, the screw thread structure of the first axial section may be formed by a first set of independent screw threads each having turns; the turns of each independent screw thread in the first set defining thread elements in the first axial section and being sequentially arranged with the turns of the other independent screw threads in the first set with adjacent turns of one of the independent screw threads of the first set being axially-spaced apart by a predetermined spacing distance which is the same for adjacent turns of the other independent screw threads in the first set; and the screw thread structure of the second axial section may be formed by (i) an independent screw thread having turns which define the thread elements of the second axial section and are axially-spaced apart by the predetermined spacing distance or essentially the predetermined spacing distance, or (ii) a second set of independent screw threads numbering less than in the first set each having turns, the turns of each independent screw thread in the second set defining thread elements in the second axial section and being sequentially arranged with the turns of the other independent screw threads in the second set with adjacent turns of each independent screw thread of the second set being axially-spaced apart by the predetermined spacing distance or essentially the predetermined spacing distance.
In an embodiment of the invention one or more of the independent screw threads of the first and second axial sections are shared by the first and second axial sections.
In an embodiment of the invention such as the one hereinafter to be described the or each independent screw thread of at least one of the first and second axial sections is a microthread, that is to say, a thread having a height which is no greater than 0.2 mm.
In an embodiment of the invention only the screw threads of the first axial section are microthreads. It could be the case, though, that the screw threads of both the first and second axial sections are microthreads.
In an embodiment of the invention the circumferentially-oriented roughness in at least one of the first and second axial sections is formed by a series of axially spaced-apart cirucmferential lines of beads. The beads in each line may be circumferentially spaced-apart.
In an embodiment of the invention such as the one hereinafter to be described the implant is a dental implant adapted for implantation in the maxilla or mandible of an edentulous patient for supporting a superstructure which presents one or more artificial teeth.
According to the invention there is further provided a method of implanting an implant into a bone tissue structure comprising the steps of providing an implant according to the invention, providing a bore-hole in the bone tissue structure and screwing the implant into the bore-hole so that the shaft is embedded in the bone tissue.